Method for producing L-amino acids using bacteria of the enterobacteriaceae family

ABSTRACT

There is disclosed a method for producing L-amino acid, for example L-threonine, L-lysine, L-histidine, L-phenylalanine, L-arginine or L-glutamic acid, using a bacterium of the Enterobacteriaceae family, wherein the bacterium has been modified to enhance an activity of D-xylose permease.

This application is a Divisional of, and claims priority under 35 U.S.C. §120 to, U.S. patent application Ser. No. 13/032,057, filed Feb. 22, 2011, which is a Continuation under 35 U.S.C. §120 of U.S. patent application Ser. No. 11/247,138, filed Oct. 12, 2005, now U.S. Pat. No. 7,915,018, which claimed priority under 35 U.S.C. §119 to Russian Application Serial No. 2004/130954, filed Oct. 22, 2004, and U.S. Provisional Patent Application Ser. No. 60/673,807, filed Apr. 22, 2005, the entireties of which are incorporated by reference. Also, the Sequence Listing filed electronically herewith is hereby incorporated by reference (File name: 2012-05-0ST_US-193D_Seq_List; File size: 15 KB; Date recorded: May 8, 2012).

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention relates to a method for producing an L-amino acid by fermentation, and more specifically to genes which aid in this fermentation. These genes are useful for the improvement of L-amino acid production, for example, for production of L-threonine, L-lysine, L-histidine, L-phenylalanine, L-arginine and L-glutamic acid.

2. Background Art

Conventionally, L-amino acids are industrially produced by fermentation methods utilizing strains of microorganisms obtained from natural sources, or mutants thereof. Typically, the microorganisms are modified to enhance production yields of L-amino acids.

Many techniques to enhance production yields of L-amino acids have been reported, including transformation of microorganisms with recombinant DNA (see, for example, U.S. Pat. No. 4,278,765). Other techniques for enhancing production yields include increasing the activities of enzymes involved in amino acid biosynthesis and/or desensitizing the target enzymes to feedback inhibition by the resulting L-amino acid (see, for example, WO 95/16042 or U.S. Pat. Nos. 4,346,170, 5,661,012 and 6,040,160).

Strains useful in production of L-threonine by fermentation are known, including strains with increased activities of enzymes involved in L-threonine biosynthesis (U.S. Pat. Nos. 5,175,107, 5,661,012, 5,705,371, and 5,939,307; EP 0219027), strains resistant to chemicals such as L-threonine and its analogs (WO 01/14525A1, EP 301572 A2, U.S. Pat. No. 5,376,538), strains with target enzymes desensitized to feedback inhibition by the produced L-amino acid or its by-products (U.S. Pat. Nos. 5,175,107 and 5,661,012), and strains with inactivated threonine degradation enzymes (U.S. Pat. Nos. 5,939,307 and 6,297,031).

The known threonine-producing strain VKPM B-3996 (U.S. Pat. Nos. 5,175,107 and 5,705,371) is presently one of the best known threonine producers. For construction of the strain VKPM B-3996, several mutations and a plasmid, described below, were introduced into the parent strain E. coli K-12 (VKPM B-7). Mutant thrA gene (mutation thrA442) encodes aspartokinase homoserine dehydrogenase I, which is resistant to feedback inhibition by threonine. Mutant ilvA gene (mutation ilvA442) encodes threonine deaminase having decreased activity which results in a decreased rate of isoleucine biosynthesis and to a leaky phenotype of isoleucine starvation. In bacteria containing the ilvA442 mutation, transcription of the thrABC operon is not repressed by isoleucine, and therefore is very efficient for threonine production. Inactivation of the tdh gene encoding threonine dehydrogenase results in prevention of threonine degradation. The genetic determinant of saccharose assimilation (scrKYABR genes) was transferred to said strain. To increase expression of the genes controlling threonine biosynthesis, plasmid pVIC40 containing the mutant threonine operon thrA442BC was introduced in the intermediate strain TDH6. The amount of L-threonine accumulated during fermentation of the strain can be up to 85 g/l.

By optimizing the main biosynthetic pathway of a desired compound, further improvement of L-amino acid producing strains can be accomplished via supplementation of the bacterium with increasing amounts of sugars as a carbon source, for example, glucose. Despite the efficiency of glucose transport by PTS, access to the carbon source in a highly productive strain still may be insufficient.

It is known that active transport of sugars and other metabolites into bacterial cells is accomplished by several transport systems. Among these, the XylE protein from E. coli is a D-xylose permease, one of two systems in E. coli responsible for the uptake of D-xylose; the other being the ATP-dependent ABC transporter XylFGH. The cloned xylE gene has been shown to complement xylE mutants in vivo (Davis, E. O. and Henderson, P. J., J. Biol. Chem., 262(29); 13928-32 (1987)). The XylE-mediated transport in whole cells is inhibited by protonophores and elicits an alkaline pH change (Lam, V. M. et al, J. Bacteriol. 143(1); 396-402 (1980)). Experiments using xylE and xylF mutants have established that XylE protein has a K_(M) of 63-169 μM for D-xylose (Sumiya. M. et al, Receptors Channels, 3(2); 117-28 (1995)). The XylE protein is a member of the major facilitator superfamily (MFS) of transporters (Griffith, J. K. et al, Curr. Opin. Cell Biol. 4(4); 684-95 (1992)) and appears to function as a D-xylose/proton symporter. The xylE gene probably constitutes a monocistronic operon whose expression is inducible by D-xylose. Imported xylose is catabolized to xylulose-5-phosphate by the action of the XylA (xylose isomerase) and XylB (xylulokinase) enzymes. Under appropriate conditions, the xylose isomerase encoded by the xylA gene also efficiently catalyzes the conversion of D-glucose to D-fructose (Wovcha, M. G. et al, Appl Environ Microbiol. 45(4): 1402-4 (1983)).

However, there has been no report to date of using a bacterium of the Enterobacteriaceae family having an enhanced activity of D-xylose permease for increasing the production of L-amino acids.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the structure of the region upstream of the xylE gene in the chromosome of E. coli and the structure of an integrated DNA fragment containing the cat gene and a hybrid P_(L-tac) promoter.

FIG. 2 shows growth curves of E. coli strains MG1655, MG1655 ΔptsHI-crr and MG1655P_(L-tac)xylE grown on medium with glucose. Legend: MG=E. coli MG1655; MG Δpts=E. coli MG1655 ΔptsHI-crr; MG Δpts P xylE=E. coli MG1655 ΔptsHI-crrP_(L-tac)xylE.

SUMMARY OF THE INVENTION

An object of present invention is to enhance the productivity of L-amino acid-producing strains and to provide a method for producing non-aromatic or aromatic L-amino acids using these strains.

This aim was achieved by finding that the increasing the expression of the xylE gene encoding D-xylose permease enhances production of L-amino acids, such as L-threonine, L-lysine, L-histidine, L-phenylalanine, L-arginine and L-glutamic acid. Thus the present invention has been completed.

It is an object of the present invention to provide an L-amino acid-producing bacterium of the Enterobacteriaceae family, wherein said bacterium has been modified to enhance an activity of D-xylose permease.

It is a further object of the present invention to provide the bacterium described above, wherein said activity of said D-xylose permease is enhanced by increasing the expression of a gene which encodes D-xylose permease.

It is a further object of the present invention to provide the bacterium described above, wherein said activity of D-xylose permease is enhanced by modifying an expression control sequence of the gene encoding D-xylose permease so that the gene expression is enhanced or by increasing the copy number of the gene encoding D-xylose permease.

It is a further object of the present invention to provide the bacterium described above, wherein said bacterium has been additionally modified to enhance an activity of glucokinase.

It is a further object of the present invention to provide the bacterium described above, wherein said bacterium has been additionally modified to enhance an activity of xylose isomerase.

It is a further object of the present invention to provide the bacterium described above, wherein said bacterium has been modified to increase the expression of the xylABFGHR locus.

It is a further object of the present invention to provide the bacterium described above, wherein the bacterium is selected from the group consisting of the genera Escherichia, Enterobacter, Erwinia, Klebsiella, Pantoea, Providencia, Salmonella, Serratia, Shigella, and Morganella.

It is a further object of the present invention to provide the bacterium described above, wherein said gene encodes a D-xylose permease selected from the group consisting of:

-   -   (A) a protein which comprises the amino acid sequence of SEQ ID         NO: 2; and     -   (B) a variant protein of the amino acid sequence shown in SEQ ID         NO: 2 which has an activity of D-xylose permease.

It is a further object of the present invention to provide the bacterium described above, wherein said gene encoding D-xylose permease comprises a DNA selected from the group consisting of:

-   -   (a) a DNA which comprises a nucleotide sequence of nucleotides 1         to 1476 in SEQ ID NO: 1; and     -   (b) a DNA which is hybridizable with a nucleotide sequence of         nucleotides 1-1476 in SEQ ID NO: 1, or a probe which can be         prepared from said nucleotide sequence under stringent         conditions, and encodes a protein having an activity of D-xylose         permease.

It is a further object of the present invention to provide the bacterium described above, wherein said stringent conditions comprise those in which washing is performed at 60° C. at a salt concentration of 1×SSC and 0.1% SDS for 15 minutes.

It is a further object of the present invention to provide the bacterium described above, wherein said bacterium is an L-threonine producing bacterium.

It is a further object of the present invention to provide the bacterium described above, wherein said bacterium has been additionally modified to enhance expression of a gene selected from the group consisting of

-   -   the mutant thrA gene which codes for aspartokinase homoserine         dehydrogenase I and is resistant to feedback inhibition by         threonine,     -   the thrB gene which codes for homoserine kinase,     -   the thrC gene which codes for threonine synthase,     -   the rhtA gene which codes for a putative transmembrane protein,         and     -   any combination thereof.

It is a further object of the present invention to provide the bacterium described above, wherein said bacterium has been modified to increase expression of said mutant thrA gene, said thrB gene, said thrC gene, and said rhtA gene.

It is a further object of the present invention to provide the bacterium described above, wherein said bacterium is an L-lysine producing bacterium.

It is a further object of the present invention to provide the bacterium described above, wherein said bacterium is an L-histidine producing bacterium.

It is a further object of the present invention to provide the bacterium described above, wherein said bacterium is an L-phenylalanine producing bacterium.

It is a further object of the present invention to provide the bacterium described above, wherein said bacterium is an L-arginine producing bacterium.

It is a further object of the present invention to provide the bacterium described above, wherein said bacterium is an L-glutamic acid producing bacterium.

It is a further object of the present invention to provide a method for producing an L-amino acid which comprises cultivating the bacterium described above in a culture medium, allowing accumulation of the L-amino acid in the culture medium, and isolating the L-amino acid from the culture medium.

It is a further object of the present invention to provide the method described above, wherein the culture medium contains xylose.

It is a further object of the present invention to provide the method described above, wherein said L-amino acid is L-threonine.

It is a further object of the present invention to provide the method described above, wherein said L-amino acid is L-lysine.

It is a further object of the present invention to provide the method described above, wherein said L-amino acid is L-histidine.

It is a further object of the present invention to provide the method described above, wherein said L-amino acid is L-phenylalanine.

It is a further object of the present invention to provide the method described above, wherein said L-amino acid is L-arginine.

It is a further object of the present invention to provide the method described above, wherein said L-amino acid is L-glutamic acid.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the present invention, “L-amino acid-producing bacterium” means a bacterium which has an ability to produce and excrete an L-amino acid in a medium, when the bacterium is cultured in the medium. The L-amino acid-producing ability may be imparted or enhanced by breeding. The term “L-amino acid-producing bacterium” as used herein also means a bacterium which is able to produce and cause accumulation of an L-amino acid in a culture medium in an amount larger than a wild-type or parental strain of E. coli, such as E. coli K-12, and preferably means that the bacterium is able to cause accumulation in a medium of an amount not less than 0.5 g/L, more preferably not less than 1.0 g/L of the target L-amino acid. The term “L-amino acids” include L-alanine, L-arginine, L-asparagine, L-aspartic acid, L-cysteine, L-glutamic acid, L-glutamine, L-glycine, L-histidine, L-isoleucine, L-leucine, L-lysine, L-methionine, L-phenylalanine, L-proline, L-serine, L-threonine, L-tryptophan, L-tyrosine, and L-valine. L-threonine, L-lysine, L-histidine, L-phenylalanine, L-arginine, and L-glutamic acid are particularly preferred.

The Enterobacteriaceae family includes bacteria belonging to the genera Escherichia, Enterobacter, Erwinia, Klebsiella, Pantoea, Photorhabdus, Providencia, Salmonella, Serratia, Shigella, Morganella, Yersinia, etc. Specifically, those classified into the Enterobacteriaceae according to the taxonomy used in the NCBI (National Center for Biotechnology Information) database (www.ncbi.nlm.nih.gov/htbinpost/Taxonomy/wgetorg?mode=Tree&id=1236&lvl=3&keep=1&sr chmode=1&unlock) can be used. A bacterium belonging to the genus of Escherichia or Pantoea is preferred.

The phrase “a bacterium belonging to the genus Escherichia” means that the bacterium is classified into the genus Escherichia according to the classification known to a person skilled in the art of microbiology. Examples of a microorganism belonging to the genus Escherichia as used in the present invention include, but are not limited to, Escherichia coli (E. coli).

The bacterium belonging to the genus Escherichia that can be used in the present invention is not particularly limited; however, e.g., bacteria described by Neidhardt, F. C. et al. (Escherichia coli and Salmonella typhimurium, American Society for Microbiology, Washington D.C., 1208, Table 1) are encompassed by the present invention.

The term “a bacterium belonging to the genus Pantoea” means that the bacterium is classified into the genus Pantoea according to the classification known to a person skilled in the art of microbiology. Some species of Enterobacter agglomerans have been recently re-classified into Pantoea agglomerans, Pantoea ananatis, Pantoea stewartii or the like, based on the nucleotide sequence analysis of 16S rRNA etc. (Int. J. Syst. Bacteriol., 43, 162-173 (1993)).

The bacterium of the present invention encompasses a strain of the Enterobacteriaceae family which has an ability to produce an L-amino acid and has been modified to enhance an activity of D-xylose permease. In addition, the bacterium of the present invention encompasses a strain of the Enterobacteriaceae family which has an ability to produce a L-amino acid and does not have a native activity of D-xylose permease, and has been transformed with a DNA fragment encoding D-xylose permease.

The phrase “activity of D-xylose permease” means an activity of transporting sugars, such as xylose and glucose, into the cell. Activity of D-xylose permease can be detected by complementation of growth delay of the bacterium which has a disrupted PTS-system of sugar transport (see, for example, the ΔptsHI-crr mutant described in the Examples) or by complementation xylE mutations in vivo (Davis, E. O. and Henderson, P. J., J. Biol. Chem., 262(29); 13928-32 (1987)).

The phrase “bacterium has been modified to enhance an activity of D-xylose permease” means that the activity per cell is higher than that of a non-modified strain, for example, a wild-type strain. Examples of such modifications include increasing the number of D-xylose permease molecules per cell, increasing the specific activity per D-xylose permease molecule, and so forth. Furthermore, a wild-type strain that may be used for comparison purposes includes, for example, Escherichia coli K-12. In the present invention, the amount of the accumulated L-amino acid, for example, L-threonine or L-arginine, can be increased in a culture medium as a result of enhancing the intracellular activity of D-xylose permease.

Enhancement of D-xylose permease activity in a bacterial cell can be attained by increasing the expression of the xylE gene encoding D-xylose permease. Any xylE gene derived from bacteria belonging to the genus Escherichia, as well as any xylE gene derived from other bacteria, such as coryneform bacteria, may be used as the D-xylose permease gene in the present invention. The xylE genes derived from bacteria belonging to the genus Escherichia are preferred.

The phrase “increasing the expression of the gene” means that the expression amount of the gene is higher than that of a non-modified strain, for example, a wild-type strain. Examples of such modification include increasing the copy number of gene(s) per cell, increasing the expression level of the gene(s), and so forth. The quantity of the copy number of a gene is measured, for example, by restricting the chromosomal DNA followed by Southern blotting using a probe based on the gene sequence, fluorescence in situ hybridization (FISH), and the like. The level of gene expression can be measured by various methods including Northern blotting, quantitative RT-PCR, and the like. Furthermore, a wild-type strain that can act as a control includes, for example, Escherichia coli K-12 or Pantoea ananatis FERM BP-6614 (US2004180404A1). Pantoea ananatis FERM BP-6614 was deposited at the National Institute of Bioscience and Human-Technology, Agency of Industrial Science and Technology, Ministry of International Trade and Industry (currently, International Patent Organism Depositary, National Institute of Advanced Industrial Science and Technology) on Feb. 19, 1998 and received an accession number of FERM P-16644. It was then converted to an international deposit under the provisions of Budapest Treaty on Jan. 11, 1999 and received an accession number of FERM BP-6614. Although this strain was identified as Enterobacter agglomerans when it was isolated, it has been re-classified into Pantoea ananatis based on nucleotide sequence analysis of 16S rRNA etc. as described above.

As a result of enhancing the intracellular activity of D-xylose permease, L-amino acid accumulation, for example L-threonine, L-lysine, L-histidine, L-phenylalanine or L-glutamic acid accumulation in a medium is increased.

The xylE gene which encodes D-xylose permease, namely D-xylose/proton symporter, from Escherichia coli has been elucidated (nucleotide numbers 4240277 to 4238802 in the sequence of GenBank accession NC_(—)000913.2, gi:49175990). The xylE gene is located between the yjbA ORF and the malG gene on the chromosome of E. coli K-12. The other xylE genes which encodes D-xylose permease have also been elucidated (AAN45595. xylose-proton sym . . . [gi:24054686], AAM41050. MFS transporter . . . [gi:21112853]: Xanthomonas campestris: XCC1759). In the present invention, the xylE gene from Escherichia coli is represented by SEQ ID NO. 1.

Upon being transported into the cell, glucose is phosphorylated by glucokinase, which is encoded by the glk gene. So, it is also desirable to modify the bacterium to have enhanced activity of glucokinase. The glk gene which encodes glucokinase of Escherichia coli has been elucidated (nucleotide numbers 2506481 to 2507446 in the sequence of GenBank accession NC_(—)000913.1, gi:16127994). The glk gene is located between the b2387 and the b2389 ORFs on the chromosome of E. coli K-12.

Under appropriate conditions, the xylose isomerase encoded by the xylA gene also efficiently catalyzes the conversion of D-glucose to D-fructose (Wovcha, M. G. et al, Appl Environ Microbiol. 45(4): 1402-4 (1983)). So, it is also desirable to modify the bacterium to have an enhanced activity of xylose isomerase. The xylA gene which encodes xylose isomerase of Escherichia coli has been elucidated (nucleotide numbers 3728788 to 3727466 in the sequence of GenBank accession NC_(—)000913.2, gi: 49175990). The xylA gene is located between the xylB and xylF genes on the chromosome of E. coli K-12.

When the culture medium contains xylose as an additional carbon source, increasing the activity of the xylose utilization enzymes is necessary. The “xylose utilization enzymes” include enzymes of xylose transport, xylose isomerization and xylose phosphorylation, and regulatory proteins. Such enzymes include xylose isomerase, xylulokinase, xylose transporters, and xylose transcriptional activator. Xylose isomerase catalyzes the reaction of isomerization of D-xylose to D-xylulose. Xylulokinase catalyzes the reaction of phosphorylation of D-xylulose using ATP yielding D-xylulose-5-phosphate and ADP. The presence of activity of xylose utilization enzymes, such as xylose isomerase and xylulokinase, is determined by complementation of corresponding xylose isomerase-negative or xylulokinase-negative E. coli mutants, respectively.

Genes coding for the above mentioned xylose utilization enzymes are located in the xylABFGHR locus on the chromosome of Escherichia coli. The gene coding for xylulokinase (EC numbers 2.7.1.17) is known and has been designated xylB (nucleotide numbers 3725546 to 3727000 in the sequence of GenBank accession NC_(—)000913.1, gi:16131435). The gene coding for the xylose binding protein transport system is known and has been designated xylF (nucleotide numbers 3728760 to 3729752 in the sequence of GenBank accession NC_(—)000913.1, gi:16131437). The gene coding for the putative ATP-binding protein of the xylose transport system is known and has been designated xylG (nucleotide numbers 3729830 to 3731371 in the sequence of GenBank accession NC_(—)000913.1, gi:16131438). The gene coding for the permease component of the ABC-type xylose transport system is known and has been designated xylH (nucleotide numbers 3731349 to 3732530 in the sequence of GenBank accession NC_(—)000913.1, gi:16131439). The gene coding for the transcriptional regulator of the xyl operon is known and has been designated xylR (nucleotide numbers 3732608 to 3733786 in the sequence of GenBank accession NC_(—)000913.1, gi:16131440).

Therefore, xylE, glk and genes of the xylABFGHR locus can be obtained by PCR (polymerase chain reaction; refer to White, T. J. et al., Trends Genet., 5, 185 (1989)) utilizing primers prepared based on the known nucleotide sequence of the gene. Genes coding for D-xylose permease of other microorganisms can be obtained in a similar manner.

The xylE gene derived from Escherichia coli is exemplified by a DNA which encodes the following protein (A) or (B):

(A) a protein which has the amino acid sequence shown in SEQ ID NO: 2; or

(B) a variant protein of the amino acid sequence shown in SEQ ID NO: 2, which has an activity of D-xylose permease.

The phrase “variant protein” as used in the present invention means a protein which has changes in the sequence, whether they are deletions, insertions, additions, or substitutions of amino acids, but still maintains the desired activity at a useful level, for example, useful for the enhanced production of an L-amino acid. The number of changes in the variant protein depends on the position or the type of amino acid residues in the three dimensional structure of the protein. It may be 2 to 30, preferably 2 to 15, and more preferably 2 to 5 for the protein (A). These changes in the variants can occur in regions of the protein which are not critical for the function of the protein. This is because some amino acids have high homology to one another so the three dimensional structure or activity is not affected by such a change. These changes in the variant protein can occur in regions of the protein which are not critical for the function of the protein. Therefore, the protein variant (B) may be one which has a homology of not less than 70%, preferably 80%, and more preferably 90%, and most preferably 95% with respect to the entire amino acid sequence of D-xylose permease shown in SEQ ID NO. 2, as long as the activity of D-xylose permease is maintained. Homology between two amino acid sequences can be determined using the well-known methods, for example, the computer program BLAST 2.0, which calculates three parameters: score, identity and similarity.

The DNA, which encodes substantially the same protein as the D-xylose permease described above, may be obtained, for example, by modifying the nucleotide sequence of DNA encoding D-xylose permease (SEQ ID NO: 1), for example, by means of the site-directed mutagenesis method so that one or more amino acid residues at a specified site involve deletion, substitution, insertion, or addition. A DNA modified as described above may be obtained by conventionally known mutation treatments. Such treatments include hydroxylamine treatment of the DNA encoding proteins of the present invention, or treatment of the bacterium containing the DNA with UV irradiation or a reagent such as N-methyl-N′-nitro-N-nitrosoguanidine or nitrous acid.

The substitution, deletion, insertion or addition of one or several amino acid residues should be conservative mutation(s) so that the activity is maintained. The representative conservative mutation is a conservative substitution. Examples of conservative substitutions include substitution of Ser or Thr for Ala, substitution of Gln, His or Lys for Arg, substitution of Glu, Gln, Lys, His or Asp for Asn, substitution of Asn, Glu or Gln for Asp, substitution of Ser or Ala for Cys, substitution of Asn, Glu, Lys, His, Asp or Arg for Gln, substitution of Asn, Gln, Lys or Asp for Glu, substitution of Pro for Gly, substitution of Asn, Lys, Gln, Arg or Tyr for His, substitution of Leu, Met, Val or Phe for Be, substitution of Be, Met, Val or Phe for Leu, substitution of Asn, Glu, Gln, His or Arg for Lys, substitution of Be, Leu, Val or Phe for Met, substitution of Trp, Tyr, Met, Ile or Leu for Phe, substitution of Thr or Ala for Ser, substitution of Ser or Ala for Thr, substitution of Phe or Tyr for Trp, substitution of His, Phe or Trp for Tyr, and substitution of Met, Be or Leu for Val.

A DNA encoding substantially the same protein as D-xylose permease can be obtained by expressing a DNA having the mutation as described above in an appropriate cell, and investigating the activity of any expressed product. A DNA encoding substantially the same protein as D-xylose permease can also be obtained by isolating a DNA, that is hybridizable with a probe having a nucleotide sequence which contains, for example, the nucleotide sequence shown as SEQ ID NO: 1, under the stringent conditions, and encodes a protein having the D-xylose permease activity. The “stringent conditions” referred to herein are conditions under which so-called specific hybrids are formed, and non-specific hybrids are not formed. For example, stringent conditions can be exemplified by conditions under which DNAs having high homology, for example, DNAs having homology of not less than 50%, preferably 80%, and still more preferably 90%, and most preferably 95% are able to hybridize with each other, but DNAs having homology lower than the above are not able to hybridize with each other. Alternatively, stringent conditions may be exemplified by conditions under which DNA is able to hybridize at a salt concentration equivalent to ordinary washing conditions in Southern hybridization, i.e., 1×SSC, 0.1% SDS, preferably 0.1×SSC, 0.1% SDS, at 60° C. Duration of washing depends on the type of membrane used for blotting and, as a rule, what is recommended by the manufacturer. For example, recommended duration of washing the Hybond™ N+ nylon membrane (Amersham) under stringent conditions is 15 minutes. Preferably, washing may be performed 2 to 3 times.

A partial sequence of the nucleotide sequence of SEQ ID NO: 1 can also be used as a probe. Probes may be prepared by PCR using primers based on the nucleotide sequence of SEQ ID NO: 1, and a DNA fragment containing the nucleotide sequence of SEQ ID NO: 1 as a template. When a DNA fragment having a length of about 300 bp is used as the probe, the hybridization conditions for washing include, for example, 50° C., 2×SSC and 0.1% SDS.

The substitution, deletion, insertion, or addition of nucleotides as described above also includes mutation which naturally occurs (mutant or variant), for example, due to variety in the species or genus of bacteria, and which contains the D-xylose permease.

“Transformation of a bacterium with DNA encoding a protein” means introduction of the DNA into a bacterium, for example, by conventional methods. Transformation of this DNA will result in an increase in expression of the gene encoding the protein of the present invention, and will enhance the activity of the protein in the bacterial cell. Methods of transformation include any known methods that have hitherto been reported. For example, a method of treating recipient cells with calcium chloride so as to increase permeability of the cells to DNA has been reported for Escherichia coli K-12 (Mandel, M. and Higa, A., J. Mol. Biol., 53, 159 (1970)) and may be used.

Methods of gene expression enhancement include increasing the gene copy number. Introducing a gene into a vector that is able to function in a bacterium of the Enterobacteriaceae family increases the copy number of the gene. Preferably, low copy vectors are used. Examples of low-copy vectors include, but are not limited to, pSC101, pMW118, pMW119, and the like. The term “low copy vector” is used for vectors, the copy number of which is up to 5 copies per cell.

Enhancement of gene expression may also be achieved by introduction of multiple copies of the gene into a bacterial chromosome by, for example, a method of homologous recombination, Mu integration or the like. For example, one act of Mu integration allows introduction of up to 3 copies of the gene into a bacterial chromosome.

Increasing the copy number of the D-xylose permease gene can also be achieved by introducing multiple copies of the D-xylose permease gene into the chromosomal DNA of the bacterium. In order to introduce multiple copies of the gene into a bacterial chromosome, homologous recombination is carried out using a sequence whose multiple copies exist as targets in the chromosomal DNA. Sequences having multiple copies in the chromosomal DNA include, but are not limited to, repetitive DNA, or inverted repeats existing at the end of a transposable element. Also, as disclosed in U.S. Pat. No. 5,595,889, it is possible to incorporate the D-xylose permease gene into a transposon, and allow it to be transferred to introduce multiple copies of the gene into the chromosomal DNA.

Enhancement of gene expression may also be achieved by placing the DNA of the present invention under the control of a potent promoter. For example, the lac promoter, the trp promoter, the trc promoter, and the P_(R) or the P_(L) promoter of lambda phage are known as potent promoters. Enhancement of gene expression may also be achieved by placing a potent terminator downstream of the DNA of the present invention. Use of a potent promoter and/or terminator can be combined with multiplication of gene copies. Alternatively, the effect of a promoter can be enhanced by, for example, introducing a mutation into the promoter to increase the transcription level of a structural gene (coding region of a gene) located downstream of the promoter. Similarly, the effect of a terminator can be enhanced by, for example, introducing a mutation into the terminator to increase the turnover of transcription of a gene located upstream of the terminator.

Furthermore, it is known that substitution of several nucleotides in the spacer between ribosome binding site (RBS) and the start codon, especially the sequences immediately upstream of the start codon, profoundly affect the mRNA translatability. For example, a 20-fold range in the expression levels was found, depending on the nature of the three nucleotides preceding the start codon (Gold et al., Annu. Rev. Microbiol., 35, 365-403, 1981; Hui et al., EMBO J., 3, 623-629, 1984). Previously, it was shown that the rhtA23 mutation is an A-for-G substitution at the −1 position relative to the ATG start codon (ABSTRACTS of 17^(th) International Congress of Biochemistry and Molecular Biology in conjugation with 1997 Annual Meeting of the American Society for Biochemistry and Molecular Biology, San Francisco, Calif. Aug. 24-29, 1997, abstract No. 457). Therefore, it may be suggested that the rhtA23 mutation enhances the rhtA gene expression and, as a consequence, increases the resistance to threonine, homoserine and some other substances transported out of cells.

Moreover, it is also possible to introduce a nucleotide substitution into a promoter or terminator region of the D-xylose permease gene on the bacterial chromosome, which results in a stronger promoter or terminator function. The alteration of the expression control sequence can be performed, for example, in the same manner as the gene substitution using a temperature-sensitive plasmid, as disclosed in International Patent Publication WO 00/18935 and Japanese Patent Application Laid-Open No. 1-215280.

Methods for preparation of plasmid DNA include, but are not limited to, digestion and ligation of DNA, transformation, selection of an oligonucleotide as a primer and the like, or other methods well known to one skilled in the art. These methods are described, for instance, in Sambrook, J., Fritsch, E. F., and Maniatis, T., “Molecular Cloning A Laboratory Manual, Second Edition”, Cold Spring Harbor Laboratory Press (1989).

The bacterium of the present invention can be obtained by introduction of the aforementioned DNAs into a bacterium which inherently has the ability to produce an L-amino acid. Alternatively, the bacterium of the present invention can be obtained by imparting an ability to produce an L-amino acid to a bacterium already containing the DNAs.

L-Threonine Producing Bacteria

Examples of parent strains for deriving the L-threonine-producing bacteria of the present invention include, but are not limited to, L-threonine-producing bacteria belonging to the genus Escherichia, such as E. coli TDH-6/pVIC40 (VKPM B-3996) (U.S. Pat. Nos. 5,175,107 and 5,705,371), E. coli NRRL-21593 (U.S. Pat. No. 5,939,307), E. coli FERM BP-3756 (U.S. Pat. No. 5,474,918), E. coli FERM BP-3519 and FERM BP-3520 (U.S. Pat. No. 5,376,538), E. coli MG442 (Gusyatiner et al., Genetika (in Russian), 1978, 14: 947-956), E. coli VL643 and VL2055 (EP 1149911 A), and the like.

The strain TDH-6 is deficient in the thrC gene, as well as being sucrose-assimilative, and the ilvA gene has a leaky mutation. This strain also has a mutation in the rhtA gene, which imparts resistance to high concentrations of threonine or homoserine. The strain B-3996 contains the plasmid pVIC40 which was obtained by inserting a thrA*BC operon which includes a mutant thrA gene into a RSF1010-derived vector. This mutant thrA gene encodes aspartokinase homoserine dehydrogenase I which has substantially desensitized feedback inhibition by threonine. The strain B-3996 was deposited in the All-Union Scientific Center of Antibiotics (Nagatinskaya Street 3-A, 117105 Moscow, Russian Federation) on Nov. 19, 1987 under accession number RIA 1867. The strain was also deposited in the Russian National Collection of Industrial Microorganisms (VKPM; Dorozhny proezd. 1, Moscow 117545, Russian Federation) under accession number B-3996.

Preferably, the bacterium of the present invention is additionally modified to enhance expression of one or more of the following genes:

-   -   the mutant thrA gene which encodes aspartokinase homoserine         dehydrogenase I resistant to feedback inhibition by threonine;     -   the thrB gene which encodes homoserine kinase;     -   the thrC gene which encodes threonine synthase;     -   the rhtA gene which encodes a putative transmembrane protein;     -   the asd gene which encodes aspartate-β-semialdehyde         dehydrogenase; and     -   the aspC gene which encodes aspartate aminotransferase         (aspartate transaminase);

The thrA gene which encodes aspartokinase homoserine dehydrogenase I of Escherichia coli has been elucidated (nucleotide positions 337 to 2799, GenBank accession no. NC_(—)000913.2, gi: 49175990). The thrA gene is located between the thrL and thrB genes on the chromosome of E. coli K-12. The thrB gene which encodes homoserine kinase of Escherichia coli has been elucidated (nucleotide positions 2801 to 3733, GenBank accession no. NC_(—)000913.2, gi: 49175990). The thrB gene is located between thrA and thrC genes on the chromosome of E. coli K-12. The thrC gene which encodes threonine synthase of Escherichia coli has been elucidated (nucleotide positions 3734 to 5020, GenBank accession no. NC_(—)000913.2, gi: 49175990). The thrC gene is located between the thrB gene and the yaaX open reading frame on the chromosome of E. coli K-12. All three genes function as a single threonine operon.

A mutant thrA gene which encodes aspartokinase homoserine dehydrogenase I resistant to feedback inhibition by threonine, as well as the thrB and thrC genes, can be obtained as one operon from the well-known plasmid pVIC40 which is present in the threonine producing E. coli VKPM B-3996. Plasmid pVIC40 is described in detail in U.S. Pat. No. 5,705,371.

The rhtA gene exists at 18 min on the E. coli chromosome close to the glnHPQ operon, which encodes components of the glutamine transport system. The rhtA gene is identical to ORF1 (ybiF gene, positions 764 to 1651, GenBank accession no. AAA218541, gi:440181) and located between the pexB and ompX genes. The unit expressing a protein encoded by the ORF1 has been designated the rhtA gene (rht: resistance to homoserine and threonine). Also, it was revealed that the rhtA23 mutation is an A-for-G substitution at position −1 with respect to the ATG start codon (ABSTRACTS of the 17^(th) International Congress of Biochemistry and Molecular Biology in conjugation with the Annual Meeting of the American Society for Biochemistry and Molecular Biology, San Francisco, Calif., Aug. 24-29, 1997, abstract No. 457, EP 1013765 A).

The asd gene of E. coli has already been elucidated (nucleotide positions 3572511 to 3571408, GenBank accession no. NC_(—)000913.1, gi:16131307), and can be obtained by PCR (polymerase chain reaction; refer to White, T. J. et al., Trends Genet., 1989, 5:185), utilizing primers based on the nucleotide sequence of the gene. The asd genes of other microorganisms can be obtained in a similar manner.

Also, the aspC gene of E. coli has already been elucidated (nucleotide positions 983742 to 984932, GenBank accession no. NC_(—)000913.1, gi:16128895), and can be obtained by PCR. The aspC genes of other microorganisms can be obtained in a similar manner.

L-Lysine Producing Bacteria

Examples of L-lysine producing bacteria belonging to the genus Escherichia include mutants having resistance to an L-lysine analogue. The L-lysine analogue inhibits growth of bacteria belonging to the genus Escherichia, but this inhibition is fully or partially desensitized when L-lysine coexists in a medium. Examples of the L-lysine analogue include, but are not limited to, oxalysine, lysine hydroxamate, S-(2-aminoethyl)-L-cysteine (AEC), γ-methyllysine, α-chlorocaprolactam and so forth. Mutants having resistance to these lysine analogues can be obtained by subjecting bacteria belonging to the genus Escherichia to a conventional artificial mutagenesis treatment. Specific examples of bacterial strains useful for producing L-lysine include Escherichia coli AJ11442 (FERM BP-1543, NRRL B-12185; see U.S. Pat. No. 4,346,170) and Escherichia coli VL611. In these microorganisms, feedback inhibition of aspartokinase by L-lysine is desensitized.

The strain WC196 may be used as an L-lysine producing bacterium of Escherichia coli. This bacterial strain was bred by conferring AEC resistance to the strain W3110, which was derived from Escherichia coli K-12. The resulting strain was designated as the Escherichia coli AJ13069 strain, and was deposited at the National Institute of Bioscience and Human-Technology, Agency of Industrial Science and Technology (currently National Institute of Advanced Industrial Science and Technology, International Patent Organism Depositary, Tsukuba Central 6, 1-1, Higashi 1-Chome, Tsukuba-shi, Ibaraki-ken, 305-8566, Japan) on Dec. 6, 1994 and received an accession number of FERM P-14690. Then, it was converted to an international deposit under the provisions of the Budapest Treaty on Sep. 29, 1995, and received an accession number of FERM BP-5252 (U.S. Pat. No. 5,827,698).

L-Histidine Producing Bacteria

Examples of parent strains for deriving the L-histidine-producing bacteria of the present invention include, but are not limited to, L-histidine-producing bacteria belonging to the genus Escherichia, such as E. coli strain 24 (VKPM B-5945, RU2003677); E. coli strain 80 (VKPM B-7270, RU2119536); E. coli NRRL B-12116-B12121 (U.S. Pat. No. 4,388,405); E. coli H-9342 (FERM BP-6675) and H-9343 (FERM BP-6676) (U.S. Pat. No. 6,344,347); E. coli H-9341 (FERM BP-6674) (EP 1085087); E. coli AI80/pFM201 (U.S. Pat. No. 6,258,554), and the like.

L-Phenylalanine Producing Bacteria

Examples of parent strains for deriving the L-phenylalanine-producing bacteria of the present invention include, but are not limited to, L-phenylalanine-producing bacteria belonging to the genus Escherichia, such as E. coli AJ12739 (tyrA::Tn10, tyrR) (VKPM B-8197); E. coli HW1089 (ATCC 55371) harboring the pheA34 gene (U.S. Pat. No. 5,354,672); E. coli MWEC101-b (KR8903681); E. coli NRRL B-12141, NRRL B-12145, NRRL B-12146, and NRRL B-12147 (U.S. Pat. No. 4,407,952). Also, as a parent strain, E. coli K-12 [W3110 (tyrA)/pPHAB (FERM BP-3566), E. coli K-12 [W3110 (tyrA)/pPHAD] (FERM BP-12659), E. coli K-12 [W3110 (tyrA)/pPHATerm] (FERM BP-12662), and E. coli K-12 [W3110 (tyrA)/pBR-aroG4, pACMAB] named AJ 12604 (FERM BP-3579) may be used (EP 488424 B1). Furthermore, L-phenylalanine-producing bacteria belonging to the genus Escherichia with an enhanced activity of the protein encoded by the yedA gene or the yddG gene may also be used (U.S. Patent Applications 2003/0148473 A1 and 2003/0157667 A1).

L-Arginine Producing Bacteria

Examples of parent strains for deriving the L-arginine-producing bacteria of the present invention include, but are not limited to, L-arginine-producing bacteria, such as E. coli strain 237 (VKPM B-7925) (U.S. Patent Application 2002/0058315 A1) and its derivative strains harboring mutant N-acetylglutamate synthase (Russian Patent Application No. 2001112869), E. coli strain 382 (VKPM B-7926) (EP 1170358 A1), an arginine-producing strain which has the argA gene encoding N-acetylglutamate synthetase introduced therein (JP 57-5693A), and the like.

L-Glutamic Acid Producing Bacteria

Examples of parent strains for deriving the L-glutamic acid-producing bacteria of the present invention include, but are not limited to, L-glutamic acid-producing bacteria belonging to the genus Escherichia, such as E. coli VL334thrC⁺ (EP 1172433). E. coli VL334 (VKPM B-1641) is an L-isoleucine and L-threonine auxotrophic strain having mutations in the thrC and ilvA genes (U.S. Pat. No. 4,278,765). A wild-type allele of the thrC gene was transferred by the method of general transduction, using bacteriophage P1 grown on wild-type E. coli K12 (VKPM B-7) cells. As a result, an L-isoleucine auxotrophic strain VL334thrC⁺ (VKPM B-8961) was obtained. This strain is able to produce L-glutamic acid.

Examples of parent strains for deriving the L-glutamic acid-producing bacteria of the present invention include, but are not limited to, mutants which are deficient in α-ketoglutarate dehydrogenase activity or mutants which have a reduced α-ketoglutarate dehydrogenase activity. Bacteria belonging to the genus Escherichia deficient in α-ketoglutarate dehydrogenase activity or having a reduced α-ketoglutarate dehydrogenase activity and methods for obtaining them are described in U.S. Pat. Nos. 5,378,616 and 5,573,945. Specifically, these strains include the following:

E. coli W3110sucA::Kmr

E. coli AJ12624 (FERM BP-3853)

E. coli AJ12628 (FERM BP-3854)

E. coli AJ12949 (FERM BP-4881)

E. coli W3110sucA::Kmr is obtained by disrupting the α-ketoglutarate dehydrogenase gene (hereinafter referred to as “sucA gene”) of E. coli W3110. This strain is completely deficient in α-ketoglutarate dehydrogenase.

Other examples of L-glutamic acid-producing bacteria include mutant strains belonging to the genus Pantoea which are deficient in α-ketoglutarate dehydrogenase activity or have a decreased α-ketoglutarate dehydrogenase activity, and can be obtained as described above. Such strains include Pantoea ananatis AJ13356. (U.S. Pat. No. 6,331,419). Pantoea ananatis AJ13356 was deposited at the National Institute of Bioscience and Human-Technology, Agency of Industrial Science and Technology, Ministry of International Trade and Industry (currently, National Institute of Advanced Industrial Science and Technology, International Patent Organism Depositary, Central 6, 1-1, Higashi 1-Chome, Tsukuba-shi, Ibaraki-ken, 305-8566, Japan) on Feb. 19, 1998 under accession no. FERM P-16645. It was then converted to an international deposit under the provisions of the Budapest Treaty on Jan. 11, 1999 and received accession no. FERM BP-6615. Pantoea ananatis AJ13356 is deficient in α-ketoglutarate dehydrogenase activity as a result of disruption of the aKGDH-E1 subunit gene (sucA). The above strain was identified as Enterobacter agglomerans when it was isolated and deposited as Enterobacter agglomerans AJ13356. However, it was recently re-classified as Pantoea ananatis on the basis of nucleotide sequencing of 16S rRNA and so forth. Although AJ13356 was deposited at the aforementioned depository as Enterobacter agglomerans, for the purposes of this specification, they are described as Pantoea ananatis.

Production of L-Amino Acids

Oxaloacetate (OAA) serves as a substrate for the reaction which results in synthesis of Thr and Lys. OAA results from a reaction of PEP with phosphoenol pyrvate carboxlase (PEPC) functioning as a catalyst. Therefore, elevation of the PEPC concentration in a cell can be very important for fermentative production of these amino acids. When using glucose as the carbon source in fermentation, glucose is internalized by the glucose-phosphontransferase (Glc-PTS) system. This system consumes PEP, and proteins in the PTS are encoded by ptsG and ptsHIcrr. During internalization, one molecule of PEP and one molecule of pyruvate (Pyr) are generated from one molecule of glucose.

An L-threonine-producing strain and an L-lysine-producing strain which have been modified to have an ability to utilize sucrose (Scr-PTS) have higher productivity of these amino acids when cultured in sucrose rather than glucose (EP 1149911 A2). It is believed that three molecules of PEP and one molecule of Pyr are generated from one molecule of sucrose by the Scr-PTS, increasing the ratio of PEP/Pyr, and thereby facilitating the synthesis of Thr and Lys from sucrose. Furthermore, it has been reported that Glc-PTS is subject to several expression controls (Postma P. W. et al., Microbiol Rev., 57(3), 543-94 (1993); Clark B. et al. J. Gen. Microbiol., 96(2), 191-201 (1976); Plumbridge J., Curr. Opin. Microbiol., 5(2), 187-93 (2002); Ryu S. et al., J. Biol. Chem., 270(6):2489-96 (1995)), and hence it is possible that the incorporation of glucose itself can be a rate-limiting step in amino acid fermentation.

Increasing the ratio of PEP/Pyr even more by increasing expression of the xylE gene in a threonine-producing strain, a lysine-producing strain, a histidine-producing strain, a phenylalanine-producing strain and/or a glutamic acid-producing strain should further increase amino acid production. Because four molecules of PEP are generated from two molecules of glucose, the ratio of PEP/Pyr is expected to be greatly improved. Due to the increased expression of the xylE gene, removal of the expression control of glc-PTS is expected.

The method for producing an L-amino acid of the present invention includes the steps of cultivating the bacterium of the present invention in a culture medium, allowing the L-amino acid to accumulate in the culture medium, and collecting the L-amino acid from the culture medium. Furthermore, the method of the present invention includes a method for producing L-threonine, L-lysine, L-histidine, L-phenylalanine, L-arginine or L-glutamic acid, including the steps of cultivating the bacterium of the present invention in a culture medium, allowing L-threonine, L-lysine, L-histidine, L-phenylalanine, L-arginine or L-glutamic acid to accumulate in the culture medium, and collecting L-threonine, L-lysine, L-histidine, L-phenylalanine, L-arginine or L-glutamic acid from the culture medium.

In the present invention, the cultivation, collection and purification of L-amino acids from the medium and the like may be performed by conventional fermentation methods wherein an L-amino acid is produced using a microorganism.

The culture medium may be either synthetic or natural, so long as the medium includes a carbon source and a nitrogen source and minerals, and if necessary, appropriate amounts of nutrients which the microorganism requires for growth. The carbon source may include various carbohydrates such as glucose, sucrose and xylose, and various organic acids. Depending on the mode of assimilation of the chosen microorganism, alcohols including ethanol and glycerol, may be used. As the nitrogen source, various ammonium salts such as ammonia and ammonium sulfate, other nitrogen compounds such as amines, a natural nitrogen source such as peptone, soybean-hydrolysate, and digested fermentative microorganism may be used. As minerals, potassium monophosphate, magnesium sulfate, sodium chloride, ferrous sulfate, manganese sulfate, calcium chloride, and the like may be used. As vitamins, thiamine, yeast extract, and the like may be used. Additional nutrients may be added to the medium, if necessary. For example, if the microorganism requires an L-amino acid for growth (L-amino acid auxotrophy), a sufficient amount of the L-amino acid may be added to the cultivation medium.

The cultivation is performed preferably under aerobic conditions such as a shaking culture, and stirring culture with aeration, at a temperature of 20 to 40° C., preferably 30 to 38° C. The pH of the culture is usually between 5 and 9, preferably between 6.5 and 7.2. The pH of the culture can be adjusted with ammonia, calcium carbonate, various acids, various bases, and buffers. Usually, a 1 to 5-day cultivation leads to accumulation of the target L-amino acid in the liquid medium.

After cultivation, solids such as cells can be removed from the liquid medium by centrifugation or membrane filtration, and then the target L-amino acid can be collected and purified by ion-exchange, concentration and/or crystallization methods.

EXAMPLES

The present invention will be more concretely explained below with reference to the following non-limiting examples.

Example 1 Substitution of the Native Promoter Region of the xylE Gene in E. Coli by Hybrid P_(L-tac) Promoter

To substitute the native promoter region of the xylE gene, a DNA fragment carrying a hybrid P_(L-tac) promoter and chloramphenicol resistance marker (Cm^(R)) encoded by the cat gene was integrated into the chromosome of the E. coli MG1655 (ATCC 700926) in place of the native promoter region by the method described by Datsenko K. A. and Wanner B. L. (Proc. Natl. Acad. Sci. USA, 2000, 97, 6640-6645) which is also called as a “Red-mediated integration” and/or “Red-driven integration”. The recombinant plasmid pKD46 (Datsenko, K. A., Wanner, B. L., Proc. Natl. Acad. Sci. USA, 2000, 97, 6640-6645) having a thermosensitive replicon was used as the donor of the phage X-derived genes responsible for the Red-mediated recombination system. Escherichia coli strain BW25113 containing the recombinant plasmid pKD46 can be obtained from the E. coli Genetic Stock Center, Yale University, New Haven, USA, the accession number of which is CGSC7630.

The hybrid P_(L-tac) promoter was synthesized chemically. The nucleotide sequence of the substituted promoter is presented in the Sequence listing (SEQ ID NO: 3). The synthesized DNA fragment containing the hybrid P_(L-tac) promoter contains a BglII recognition site at the 5′-end thereof, which is necessary for further joining to the cat gene and 36 nucleotides homologous to the 5′-terminus of the xylE gene introduced for further integration into the bacterial chromosome.

A DNA fragment containing a Cm^(R) marker encoded by the cat gene was obtained by PCR using the commercially available plasmid pACYC184 (GenBank/EMBL accession number X06403, “Fermentas”, Lithuania) as the template, and primers P1 (SEQ ID NO: 4) and P2 (SEQ ID NO: 5). Primer P1 contains a BglII recognition site at the 5′-end thereof, which is necessary for further joining to the hybrid P_(L-tac) promoter and primer P2 contains 36 nucleotides homologous to the region located 217 bp upstream of the start codon of the xylE gene, which was introduced into the primer for further integration into the bacterial chromosome.

PCR was provided using the “TermoHybaid PCR Express” amplificator. The reaction mixture (total volume—50 μl) consisted of 5 μl of 10×PCR-buffer with 15 mM MgCl₂ (“Fermentas”, Lithuania), 200 μM each of dNTP, 25 μmol each of the exploited primers and 1 U of Taq-polymerase (“Fermentas”, Lithuania). Approximately 5 ng of the plasmid DNA was added into the reaction mixture as a template DNA for the PCR amplification. The temperature profile was the following: initial DNA denaturation for 5 min at 95° C., followed by 25 cycles of denaturation at 95° C. for 30 sec, annealing at 55° C. for 30 sec, elongation at 72° C. for 30 sec; and the final elongation for 7 min at +72° C. Then, the amplified DNA fragment was purified by agarose gel-electrophoresis, extracted using “GenElute Spin Columns” (“Sigma”, USA) and precipitated by ethanol.

Each of the two above-described DNA fragments was treated with BglII restrictase and ligated. The ligation product was amplified by PCR using primers P2 (SEQ ID NO: 5) and P3 (SEQ ID NO: 6). Primer P3 contains 36 nucleotides at 5′-end thereof which are homologous to the 5′-terminus of the xylE gene introduced for further integration into the bacterial chromosome.

The amplified DNA fragment was purified by agarose gel-electrophoresis, extracted using “GenElute Spin Columns” (“Sigma”, USA) and precipitated by ethanol. The obtained DNA fragment was used for electroporation and Red-mediated integration into the bacterial chromosome of the E. coli MG1655/pKD46.

MG1655/pKD46 cells were grown overnight at 30° C. in the liquid LB-medium with the addition of ampicillin (100 μg/ml), then diluted 1:100 with the SOB-medium (Yeast extract, 5 g/l; NaCl, 0.5 g/l; Tryptone, 20 g/l; KCl, 2.5 mM; MgCl₂, 10 mM) with the addition of ampicillin (100 μg/ml) and L-arabinose (10 mM) (arabinose is used for inducing the plasmid encoding genes of the Red system) and grown at 30° C. to reach the optical density of the bacterial culture OD₆₀₀=0.4-0.7. Grown cells from 10 ml of the bacterial culture were washed 3 times with the ice-cold de-ionized water, followed by suspending in 100 μl of the water. 10 μl of DNA fragment (100 ng) dissolved in the de-ionized water was added to the cell suspension. The electroporation was performed by “Bio-Rad” electroporator (USA) (No. 165-2098, version 2-89) according to the manufacturer's instructions. Shocked cells were added to 1-ml of SOC medium (Sambrook et al, “Molecular Cloning A Laboratory Manual, Second Edition”, Cold Spring Harbor Laboratory Press (1989)), incubated 2 hours at 37° C., and then were spread onto L-agar containing 25 μg/ml of chloramphenicol. Colonies grown within 24 hours were tested for the presence of Cm^(R) marker, instead of the native promoter region of the xylE gene by PCR using primers P4 (SEQ ID NO: 7) and P5 (SEQ ID NO: 8). For this purpose, a freshly isolated colony was suspended in 20 μl water and then 1 μl of obtained suspension was used for PCR. The following temperature profile was used: initial DNA denaturation for 10 min at 95° C.; then 30 cycles of denaturation at 95° C. for 30 sec, annealing at 55° C. for 30 sec and elongation at 72° C. for 1 min; the final elongation for 7 min at 72° C. A few Cm^(R) colonies tested contained the desired ˜2000 bp DNA fragment, confirming the presence of Cm^(R) marker DNA instead of 192 bp native promoter region of xylE gene (see FIG. 1). One of these strains was cured from the thermosensitive plasmid pKD46 by culturing at 37° C. and the resulting strain was named as E. coli MG1655P_(L-tac)xylE.

Example 2 Effect of Increasing the xylE Gene Expression on Growth of an E. Coli Strain Having a Disrupted PTS Transport System

To show the effect of enhanced expression of the xylE gene on growth of an E. coli strain, the E. coli strain having a disrupted PTS transport system was constructed.

For that purpose, the DNA fragment carrying kanamycin resistance marker (Km^(R)) was integrated into the chromosome of the E. coli MG1655/pKD46 in place of the ptsHI-crr operon by the method described by Datsenko K. A. and Wanner B. L. (Proc. Natl. Acad. Sci. USA, 2000, 97, 6640-6645) which is also called as a “Red-mediated integration” and/or “Red-driven integration”, also described in Example 1.

The ptsHI-crr operon has been elucidated (nucleotide numbers 2531786 to 2532043, 2532088 to 2533815 and 2533856 to 2534365 for ptsH, ptsI and crr genes, respectively, in the sequence of GenBank accession NC_(—)000913.2, gi: 49175990). The ptsHI-crr operon is located between cysK and pdxK genes on the chromosome of E. coli K-12.

A DNA fragment carrying the Km^(R) gene was obtained by PCR using the commercially available plasmid pUC4KAN (GenBank/EMBL accession number X06404, “Fermentas”, Lithuania) as the template and primers P6 (SEQ ID NO: 9) and P7 (SEQ ID NO: 10). Primer P6 contains 36 nucleotides homologous to the 5′-terminus of the ptsH gene and primer P7 contains 36 nucleotides homologous to the 3′-terminus of the crr gene. These sequences were introduced into primers P6 and P7 for further integration into the bacterial chromosome.

PCR was conducted as described in Example 1.

Then, the amplified DNA fragment was concentrated by agarose gel-electrophoresis, extracted from the gel by the centrifugation through “GenElute Spin Columns” (“Sigma”, USA) and precipitated by ethanol. The obtained DNA fragment was used for electroporation and Red-mediated integration into the bacterial chromosome of the E. coli MG1655/pKD46 as described in Example 1, except that cells were spread after electroporation onto L-agar containing 50 μg/ml of kanamycin.

Colonies grown within 24 hours were tested for the presence of Km^(R) marker instead of ptsHI-crr operon by PCR using primers P8 (SEQ ID NO: 11) and P9 (SEQ ID NO: 12). For this purpose, a freshly isolated colony was suspended in 20 μl water and then 1 μl of the resulting suspension was used for PCR. PCR conditions were as described in Example 1. A few Km^(R) colonies tested contained the desired ˜1300 bp DNA fragment, which confirmed the presence of Km^(R) gene in the place of the ptsHI-crr operon. One of the obtained strains was cured from thermosensitive plasmid pKD46 by culturing at 37° C. and the resulting strain was named E. coli MG1655 ΔptsHI-crr.

Then, the DNA fragment from the chromosome of the above-mentioned E. coli MG1655P_(L-tac)xylE was transferred to E. coli MG1655 ΔptsHI-crr by P1 transduction (Miller, J. H. (1972) Experiments in Molecular Genetics, Cold Spring Harbor Lab. Press, Plainview, N.Y.) giving the strain MG1655 ΔptsHI-crr P_(L-tac)xylE.

The ability to grow on the minimal Adams with glucose (4%) as a carbon source was checked for the four E. coli strains MG1655, MG1655 ΔptsHI-crr and MG1655 ΔptsHI-crr P_(L-tac)xylE. As seen in FIG. 2, E. coli MG1655 ΔptsHI-crr did not grow well (μ˜0.06) on the minimal Adams medium containing glucose. Increasing the xylE gene expression significantly enhanced the growing characteristics of recipient strains on the minimal Adams medium containing glucose.

Example 3 Effect of Increasing the xylE Gene Expression on Threonine Production

To test the effect of enhanced expression of the xylE gene which is under the control of P_(L-tac) promoter on threonine production, DNA fragments from the chromosome of the above-described E. coli MG1655P_(L-tac)XylE were transferred to the threonine-producing E. coli strain VKPM B-3996 by P1 transduction (Miller, J. H. (1972) Experiments in Molecular Genetics, Cold Spring Harbor Lab. Press, Plainview, N.Y.). The strain VKPM B-3996 was deposited in Russian National Collection of Industrial Microorganisms (VKPM) (Russia, 117545 Moscow, 1^(st) Dorozhny proezd, 1) on Apr. 7, 1987 under the accession number B-3996.

Both E. coli strains B-3996 and B-3996P_(L-tac)XylE were grown for 18-24 hours at 37° C. on L-agar plates. To obtain a seed culture, the strain was grown on a rotary shaker (250 rpm) at 32° C. for 18 hours in 20×200 mm test tubes containing 2 ml of L-broth with 4% sucrose. Then, the fermentation medium was inoculated with 0.21 ml (10%) seed material. The fermentation was performed in 2 ml of minimal medium for fermentation in 20×200 mm test tubes. Cells were grown for 48 hours at 32° C. with shaking at 250 rpm.

After cultivation, the amount of accumulated L-threonine in the medium was determined by paper chromatography using following mobile phase:butanol:acetic acid:water=4:1:1 (v/v). A solution (2%) of ninhydrin in acetone was used as a visualizing reagent. A spot containing L-threonine was cut off, L-threonine was eluted in 0.5% water solution of CdCl₂, and the amount of L-threonine was estimated spectrophotometrically at 540 nm. The results are presented in Table 1. Threonine production was improved due to introduction of P_(L-tac)xylE.

The composition of the fermentation medium (g/l) is as follows:

Glucose 80.0 (NH₄)₂SO₄ 22.0 NaCl 0.8 KH₂PO₄ 2.0 MgSO₄ 7H₂O 0.8 FeSO₄ 7H₂O 0.02 MnSO₄ 5H₂O 0.02 Thiamine HCl 0.0002 Yeast extract 1.0 CaCO₃ 30.0 Glucose and magnesium sulfate are sterilized separately. CaCO₃ is dry-heat sterilized at 180° C. for 2 hours. The pH is adjusted to 7.0. Antibiotic is introduced into the medium after sterilization.

Example 4 Effect of Increasing the xylE Gene Expression on L-Lysine Production

The whole nucleotide sequence of the chromosomal DNA of E. coli W3110 is already known (Science, 277, 1453-1474 (1997)). Based on the reported nucleotide sequence, primers were synthesized and the xylE gene was amplified by the PCR method as follows.

The chromosomal DNA was prepared by the conventional method (Sambrook, J., Fritsch E. F. and Maniatis T. (1989): Molecular Cloning: A laboratory manual, 2nd ed. Cold Spring Harbor Laboratory Press, Cold Spring Harbor, N.Y.). The primer 10 (SEQ ID NO: 13) was designed as a sequence in which an EcoRI recognition site is added to the 5′-terminal of the sequence 7479-7508 of the accession No. AE000476, and the primer 11 (SEQ ID NO: 14) was designed as a sequence complementary to the sequence in which a SalI recognition site is added to the 5′-terminal of the sequence 8963-8992 of the accession No. AE000476. By using these primers, the xylE gene was amplified according to the standard conditions as described in “PCR protocols. Current methods and applications” (White, B. A., ed., Humana Press, Totowa, N.J., 1993).

The PCR product was purified by a conventional method. The product was digested with restriction enzymes SalI and EcoRI, and using a ligation kit, ligated to the vector pSTV29 which had been treated with the same restriction enzymes. Competent cells of E. coli JM109 were transformed with the ligation product (Sambrook, J., Fritsch E. F. and Maniatis T. (1989) Molecular Cloning: A laboratory manual, 2nd ed. Cold Spring Harbor Laboratory Press, Cold Spring Harbor, N.Y.), and the cells were plated on an L-plate (Bacto-trypton: 10 g/l, yeast extract: 5 g/l, NaCl: 5 g/l, agar: 15 g/l, pH 7.0) containing 10 μg/ml of IPTG (isopropyl-β-D-thiogalactopyranoside), 40 μg/ml of X-Gal (5-bromo-4-chloro-3-indolyl-β-D-galactoside) and 50 μg/ml of chloramphenicol and were cultured overnight. White colonies appeared and were picked up and isolated, and transformants were thus obtained. Plasmids were prepared from the transformants by the alkali-extraction method, and the plasmid pSTV29-xylE was obtained in which the xylE gene is linked to the lac promoter in the forward direction.

E. coli WC196 was used as an L-lysine producing strain belonging to the genus Escherichia.

WC196 was transformed with either the plasmid pSTV29-xylE or the vector pSTV29 and WC196/pSTV29-xylE and WC196/pSTV29 were obtained. Each of these strains was cultured in the L-medium containing 50 mg/l of chloramphenicol at 37° C. until the final OD at 600 nm reached around 0.6. Then an equal volume of 40% glycerol solution was added to the culture, and the mixture was dispensed in an appropriate volume and stocked at −80° C. This is hereafter called as a “glycerol stock”.

In order to verify the effect of enhancing the xylose permease activity under L-lysine producing conditions, WC196 was transformed with the plasmids pSTV29-xylE and pCABD2 in accordance with the procedure as stated above. pCABD2 is a plasmid comprising a dapA gene coding for a dihydrodipicolinate synthase having a mutation which desensitizes feedback inhibition by L-lysine, a lysC gene coding for aspartokinase III having a mutation which desensitizes feedback inhibition by L-lysine, a dapB gene coding for a dihydrodipicolinate reductase gene, and a ddh gene coding for diaminopimelate dehydrogenase (U.S. Pat. No. 6,040,160). As a control, WC196 was transformed with the plasmids pSTV29 and pCABD2. Each of the obtained transformants was cultured in the L-medium containing 50 mg/l of chloramphenicol and 20 mg/l of streptomycin at 37° C. until the final OD at 600 nm reached around 0.6. Then an equal volume of 40% glycerol solution was added to the culture, and the mixture was dispensed in an appropriate volume and stocked at −80° C.

The glycerol stock of each of WC196/pSTV29-xylE and WC196/pSTV29 was melted and 100 μl each was evenly plated on an L-plate containing 50 mg/l of chloramphenicol, and cultured at 37° C. for 24 hours. In addition, each of WC196/(pCABD2, pSTV29-xylE) and WC196/(pCABD2, pSTV29) was evenly plated on an L-plate containing 50 mg/l of chloramphenicol and 20 mg/l of streptomycin, and cultured at 37° C. for 24 hours. About one-eighth the amount of cells on the plate was inoculated into 20 ml of the fermentation medium containing the required drug(s) in a 500 ml-flask. The cultivation was carried out at 37° C. for 16 hours by using a reciprocal shaker at the agitation speed of 115 rpm. After the cultivation, the amounts of L-lysine and residual glucose in the medium were measured by a known method (Biotech-analyzer AS210, manufactured by Sakura Seiki Co.). And then the yield of L-lysine relative to the consumed glucose was calculated for each of the strains.

The composition of the fermentation medium (g/l) is as follows:

Glucose 40 (NH₄)₂SO₄ 24 K₂HPO₄ 1.0 MgSO₄ × 7H₂O 1.0 FeSO₄ × 7H₂O 0.01 MnSO₄ × 5H₂O 0.01 Yeast extract 2.0 pH is adjusted to 7.0 by KOH and the medium is autoclaved at 115° C. for 10 min. Glucose and MgSO₄×7H₂O are sterilized separately. 30 g/l of CaCO₃, which has been dry-heat sterilized at 180° C. for 2 hours, is added.

The results are shown in Table 2. WC196/pSTV29-xylE accumulated a higher amount of L-lysine as compared with WC196/pSTV29, in which the expression amount of xylose permease is not increased. In addition, it was observed that enhancing the xylose permease activity improves the accumulation and yield of L-lysine also in WC196/pCABD2, which produces L-lysine in a higher amount.

Example 5 Effect of the Increasing the xylE Gene Expression on L-Arginine Production

To test the effect of enhanced expression of the xylE gene which is under the control of P_(L-tac) promoter on arginine production, DNA fragments from the chromosome of the above-described E. coli MG1655P_(L-tac)xylE were transferred to the arginine-producing E. coli strain 382 by P1 transduction (Miller, J. H. (1972) Experiments in Molecular Genetics, Cold Spring Harbor Lab. Press, Plainview, N.Y.). The strain 382 has been deposited in the Russian National Collection of Industrial Microorganisms (VKPM) (Russia, 117545 Moscow, 1^(st) Dorozhny proezd, 1) on Apr. 10, 2000 under accession number VKPM B-7926.

The resulting strain 382 P_(L-tac)xylE and parent strain 382 were each cultivated at 32° C. for 18 hours in 2 ml of LB nutrient broth, and 0.3 ml of the obtained culture was inoculated into 2 ml of fermentation medium in a 20×200 mm test tube, and cultivated at 32° C. for 48 hours on a rotary shaker.

After the cultivation, the amount of L-arginine accumulated in the medium was determined by paper chromatography using following mobile phase:butanol:acetic acid:water=4:1:1 (v/v). A solution (2%) of ninhydrin in acetone was used as a visualizing reagent. A spot containing L-arginine was cut off, L-arginine was eluted in 0.5% water solution of CdCl₂, and the amount of L-arginine was estimated spectrophotometrically at 540 nm.

The composition of the fermentation medium (g/l) is as follows:

Glucose 48.0 (NH4)₂SO₄ 35.0 KH₂PO₄ 2.0 MgSO₄ × 7H₂O 1.0 Thiamine HCl 0.0002 Yeast extract 1.0 L-isoleucine 0.1 CaCO3 5.0 Glucose and magnesium sulfate are sterilized separately. CaCO₃ is dry-heat sterilized at 180° C. for 2 hours. pH is adjusted to 7.0.

The results of 10 independent experiments are presented in Table 3. It can be seen from the Table 3, strain 382 P_(L-tac)XylE accumulated a higher amount of L-arginine as compared with strain 382, in which the expression amount of D-xylose permease is not increased.

Example 6 Production of L-Histidine by L-Histidine Producing Bacterium from Fermentation of a Mixture of Glucose and Xylose

The L-histidine-producing E. coli strain 80 was used for production of L-histidine by fermentation of a mixture of glucose and xylose. E. coli strain 80 (VKPM B-7270) is described in detail in Russian patent RU2119536.

To test the effect on histidine production of enhanced expression of the xylE gene which is under the control of P_(L-tac) promoter, the DNA fragments from the chromosome of the above-described E. coli MG1655P_(L-tac)xylE were transferred to histidine-producing E. coli strain 80 by P1 transduction (Miller, J. H. (1972) Experiments in Molecular Genetics, Cold Spring Harbor Lab. Press, Plainview, N.Y.). Transformation of strain 80 and the resulting strain 80 P_(L-tac)XylE with the pMW119mod-xylA-R plasmid was performed by ordinary methods, yielding strains 80/pMW119mod-xylA-R and 80 P_(L-tac)xylE/pMW119mod-xylA-R. Cloning of the xylABFGHR locus from the chromosome of E. coli strain MG1655 is described in the Russian patent application RU2005106720.

To obtain the seed culture, both strains 80/pMW119mod-xylA-R and 80 P_(L-tac)XylE/pMW119mod-xylA-R, were grown on a rotary shaker (250 rpm) at 27° C. for 6 hours in 40 ml test tubes (Ø18 mm) containing 2 ml of L-broth with 1 g/l of streptomycin and 100 mg/l ampicillin. Then, 2 ml (5%) of seed material was inoculated into the fermentation medium. Fermentation was carried out on a rotary shaker (250 rpm) at 27° C. for 50 hours in 40 ml test tubes containing 2 ml of fermentation medium.

After cultivation, the amount of L-histidine which had accumulated in the culture medium was determined by paper chromatography. The composition of the mobile phase is the following: butanol:acetate:water=4:1:1 (v/v). A solution (0.5%) of ninhydrin in acetone was used as a visualizing reagent. The results are presented in Table 4.

The composition of the fermentation medium (g/l) is as follows:

Carbohydrates (total) 100.0 Mameno (Soybean hydrolysate) 0.2 of total nitrogen L-proline 0.8 (NH₄)₂SO₄ 25.0 K₂HPO₄ 2.0 MgSO₄ × 7H₂O 1.0 FeSO₄ × 7H₂O 0.01 MnSO₄ × 5H₂O 0.01 Thiamine HCl 0.001 Betaine 2.0 CaCO₃ 6.0 Streptomycin 1.0 Carbohydrates (glucose, xylose), L-proline, betaine and magnesium sulfate are sterilized separately. CaCO₃ is dry-heat sterilized at 110° C. for 30 min. pH is adjusted to 6.0 by KOH before sterilization.

It can be seen from Table 4 that strain 80 P_(L-tac)XylE/pMW119mod-xylA-R caused accumulation of a higher amount of L-histidine in the medium containing glucose and xylose mixture as compared with strain 80/pMW119mod-xylA-R, in which the expression of D-xylose permease is not increased.

While the invention has been described in detail with reference to preferred embodiments thereof, it will be apparent to one skilled in the art that various changes can be made, and equivalents employed, without departing from the scope of the invention. Each of the aforementioned documents is incorporated by reference herein in its entirety.

TABLE 1 Strain OD₅₄₀ Threonine, g/l B3996P_(L-tac)xylE 21.6 26.9 19.5 24.0 21.3 25.9 23.4 26.4 19.7 24.6 22.0 29.4 18.9 28.4 20.0 25.9 22.6 26.4 20.8 27.1 20.5 26.9 19.5 25.9 19.5 25.4 20.2 23.6 20.7 28.7 20.2 28.9 19.4 29.2 20.5 29.0 20.2 29.5 20.5 29.2 20.6 ± 1.1 27.1 ± 1.9 B-3996 16.5 21.0 (control) 16.0 19.8 16.0 19.0 14.4 17.5 15.5 19.0 14.7 18.7 16.6 20.6 16.5 21.0 14.7 16.0 15.7 20.3 15.7 ± 0.8 19.3 ± 1.6

TABLE 2 L-Lysine Yield from Strain HCl, g/l glucose (%) WC196/pSTV29 0.5 2.3 WC196/pSTV29-xylE 1.0 3.5 WC196/pCABD2, pSTV29 1.8 32.4 WC196/pCABD2, pSTV29-xylE 4.0 38.7

TABLE 3 Strain OD₅₄₀ Arginine, g/l 382 17.0 4.9 16.8 4.6 16.8 4.9 19.5 6.2 21.1 5.3 16.3 4.7 16.1 4.6 17.1 4.6 17.6 4.9 17.0 4.7 17.5 ± 1.6 4.9 ± 0.5 382 P_(L-tac)xylE 18.0 9.6 17.8 11.9  20.3 7.4 19.2 8.6 21.3 7.7 20.1 7.0 19.7 6.6 20.6 7.6 20.0 6.9 20.6 8.4 19.8 ± 1.1 8.2 ± 1.6

TABLE 4 Residual Residual Amount of CT, Growth, glucose, xylose, histidine, Strain hours A₄₅₀ % % g/l 80/pMW119mod- 0 — 100 100 — xylA-R 16 15 85.6 92.3 0.3 + 0.01 24 48 44.3 79.8 1.1 + 0.08 40 67 0.86 14.7 5.9 + 0.05 48 66 <0.5 0.7 6.5 + 0.3  80 P _(1-tac)XylE/ 0 — 100 100 — pMW119mod- 16 15 90.9 89.3 0.3 + 0.01 xylA-R 24 25 78.6 72.9 0.7 + 0.03 40 69 26.3 <0.5 6.3 + 0.3  48 66 2.5 <0.5 7.4 + 0.3  

We claim:
 1. A method for producing L-amino acid, which comprises cultivating a bacterium of Enterobacteriaceae family in a culture medium to cause accumulation of the L-amino acid in the culture medium, and isolating the L-amino acid from the culture medium, wherein said bacterium has been modified to enhance an activity of D-xylose permease and activities of xylose utilization proteins encoded by xylABFGHR locus by a method selected from the group consisting of: a) increasing the copy number of a gene encoding D-xylose permease and the xylABFGHR locus, b) placing the gene encoding D-xylose permease and the xylABFGHR locus under the control of a potent promoter, and c) combinations thereof, wherein the carbon source in said culture medium consists of glucose; and wherein said D-xylose permease and said xylose utilization proteins encoded by xylABFGHR locus are both derived from a bacteria of the Enterobacteriaceae family.
 2. The method according to claim 1, wherein said gene encoding D-xylose permease is selected from the group consisting of: (A) a protein comprising the amino acid sequence of SEQ ID NO:2; and (B) a protein comprising an amino acid sequence which is at least 95% homologous to the sequence of SEQ ID NO:2 and has an activity of D-xylose permease.
 3. The method according to claim 1, wherein said gene encoding D-xylose permease is selected from the group consisting of: a) a DNA comprising nucleotides 1 to 1476 in SEQ ID NO:1; and b) a DNA which is hybridizable with nucleotides 1 to 1476 in SEQ ID NO:1 under stringent conditions comprising washing at 60° C. at a salt concentration of 1×SSC and 0.1% SDS for 15 minutes, and said DNA encodes a protein having an activity of D-xylose permease.
 4. The method according to claim 1, wherein said bacterium is an L-threonine producing bacterium.
 5. The method according to claim 4, wherein said bacterium has been additionally modified to enhance expression of a gene selected from the group consisting of the mutant thrA gene which codes for aspartokinase homoserine dehydrogenase I and is resistant to feedback inhibition by threonine, the thrB gene which codes for homoserine kinase, the thrC gene which codes for threonine synthase, the rhtA gene which codes for a putative transmembrane protein, and combinations thereof.
 6. The method according to claim 5, wherein said bacterium has been modified to increase expression of said mutant thrA gene, said thrB gene, said thrC gene, and said rhtA gene.
 7. The method according to claim 1, wherein said bacterium is Escherichia coli.
 8. The method according to claim 1, wherein said bacterium is Pantoea ananatis. 